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RE: [Xen-users] Slow TCP performance between Windows Vista and XenPV-on-

To: "Fischer, Anna" <anna.fischer@xxxxxx>, <xen-users@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-users] Slow TCP performance between Windows Vista and XenPV-on-HVM guest
From: "James Harper" <james.harper@xxxxxxxxxxxxxxxx>
Date: Sat, 12 Jun 2010 09:09:38 +1000
Delivery-date: Fri, 11 Jun 2010 16:10:57 -0700
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Thread-topic: [Xen-users] Slow TCP performance between Windows Vista and XenPV-on-HVM guest
> I am running a Xen HVM guest with netfront PV drivers. This is running
> SP3 inside the guest. The Dom0 is also SLES10 SP3.
> Now I am trying to communicate from that HVM guest to a Windows Visa
or also
> Windows 7 machine and I am getting really poor TCP performance. When
> on the network traffic, I can see that no packets are dropped or
missing or
> anything, but what happens is that the Vista machine does not seem to
send an
> ACK back to the Xen guest, and then the Xen guest is doing a
retransmit after
> a while.

Are you capturing packets on the windows machine or on the Dom0?

If you are using tcpdump on dom0, make sure you use '-s0' so that you
capture the entire packup, and possibly '-v' as well. Without capturing
the entire packet, tcpdump can't tell you if the checksum is correct or
not. Even if the checksum is incorrect on Dom0 it doesn't necessarily
tell you that there is a problem though. A bad checksum on received
packets on the windows machine would definitely suggest a problem


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